Radiation dosimetry involves the quantitative measurement of the physical changes that occur in matter upon exposure to ionizing radiation such as beta and alpha particles, neutral particles such as neutrons, and electromagnetic radiation such as X-rays and gamma rays. It is an important aspect of numerous civilian and military applications, including individual and environmental monitoring, retrospective and accident dosimetry, radiation therapy dosimetry, diagnostic radiology and nuclear medicine dosimetry, and neutron, particle and space dosimetry.
Thermoluminescence Dosimetry
Thermoluminescence dosimetry (TLD) has been the most widely used technique for measuring radiation doses for more than 50 years. See A. J. J. Bos, “High sensitivity Thermoluminescence Dosimetry,” Nuclear Instruments and Methods in Research B 184, 3-28 (2001). Thermoluminescent dosimeters, also often referred to as thermoluminescent phosphors, operate on the principle that radiation interactions with a material such as a crystal or a glass cause ionization of atoms in the material creating free electrons and holes. Impurities deliberately added to the dosimeter form trapping centers that trap the free electrons in metastable states that can persist for long periods of time, up to many months or years. When the dosimeter is heated, trapped electrons are released and recombine with holes, and light is emitted at a characteristic wavelength that is associated with the luminescent impurity.
There are several types of TLD readers. One type utilizes a metal planchet that can be heated at a constant rate using electric current. Another type of reader uses a hot gas jet to heat the dosimeter. Still another type of reader utilizes a laser to heat a substrate that transfers heat to the TLD dosimeter. In each of these readers, the TLD dosimeter is heated at a constant rate and the emitted light, or “luminescence” from the dosimeter is detected by a photomultiplier tube. The amount of light emitted is proportional to the amount of radiation that was absorbed, with peaks in the detected luminescence being observed over specific temperature ranges.
A plot of this thermoluminescence vs. temperature is referred to as a “glow curve” such as the curve shown in FIG. 1. This glow curve can be analyzed to determine the absorbed dose. The temperatures associated with the glow peaks are related to the depths of the traps; the deeper the trap, the higher the temperature required to release the trapped electron. Peak temperatures of approximately 200 C are generally required to permit applications, such as personal dosimetry, that require relatively long-lived traps (low signal fade). This is a typical glow curve for a LiF dosimeter chip that has been exposed to radiation. The maximum peak intensity and the integrated area under the peaks indicate the level of radiation exposure that the dosimeter has undergone.
A typical TLD dosimeter is fabricated in small pieces, or “chips.” A typical chip consists of a collection of microcrystals that are pressed into the form of a round, 3-4 mm diameter disk, or a 3-4 mm square piece that is between 0.1 and 0.7 mm thick. See, e.g., dosimeters produced by Thermo Fisher Scientific, Inc., described at http://www.thermoscientific.com/en/product/tld-100-thermoluminescent-dosimetry-material.html. These materials are also generally available as powders with grain sizes of 70 micrometers to 180 micrometers. The most widely used material for TLD dosimetry is lithium fluoride (LiF), containing magnesium (Mg) and titanium (Ti) impurities. A more sensitive variation of this dosimeter contains small amounts of copper (Cu) and phosphorous (P) (LiF:Mg,Cu,P).
The sensitivity, and thus the performance, of these dosimeters can be dramatically influenced by the thermal history of the material. See S. W. S. McKeever, M. Moscovitch, and P. D. Townsend, Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing, pp. 38-39, 50-51, 83-84, 97-98, 101, 104-105, 1995 (McKeever 1995) As a result, a chip must be subjected to a rigorous thermal annealing protocol after its thermoluminescence signal has been read out in order to recover the chip's initial sensitivity. See C. A. Carlsson, “Thermoluminescence of LiF: Dependence of Thermal History,” Phys. Med. Biol. 14(1), 107-118 (1969). In the case of LiF:Mg, Cu, P chips, heating above 250° C. for a period of time longer than about 30 minutes can permanently change their sensitivity. See McKeever 1995, supra, pp. 56-57.
Even if their sensitivity isn't permanently damaged, many TLD materials exhibit a decrease in their thermoluminescence efficiency as the temperature is increased. This occurs because nonradiative decay processes become more likely at higher temperatures. This results in thermal quenching of the thermoluminescence emission at elevated temperature and an overall reduction of the sensitivity of the TLD material. Thermal quenching can also cause the TL sensitivity to be highly dependent on the heating rate, with the TL sensitivity potentially being significantly reduced as a result of high heating rates. This can be a significant problem for high-throughput, commercial TLD readers. The ability to improve throughput by speeding up the reading rate is limited because large losses in the sensitivity of the TLD cannot be tolerated.
In addition, heating a TLD phosphor for readout irreversibly depopulates the trapped charges in the phosphor, thereby permanently erasing the stored dose information.
Optically Stimulated Luminescence
Metastable populated traps present in many TLD phosphors that have been previously exposed to radiation can also be depopulated optically in an optically stimulated luminescence (OSL) process that is analogous to thermal depopulation, where phosphors that exhibit optically stimulated luminescence undergo recombination luminescence that is observed after optical stimulation causes depopulation of the trapped charges, rather than depopulation by thermal heating.
Optical stimulation as understood by those skilled in the art is a phenomenon in which trapped charges such as trapped electrons in a phosphor are optically depopulated from their traps and subsequently recombine with holes at recombination sites in the material. See S. W. S. McKeever, “Optically Stimulated Luminescence Dosimetry,” Nuclear Instruments and Methods in Research B 184, 29-54 (2001) (McKeever 2001). As with thermally stimulated luminescence, a fraction of those electrons and holes recombine radiatively, producing luminescence from the OSL phosphor.
The response time of a dosimeter readout in response to optical stimulation is determined by the lifetime of the free electrons after they are depopulated and the decay time of the luminescence, while the response time for thermal stimulation is determined by the heating rate applied to the TL dosimeter. In some materials the mechanism for optical stimulation of traps is the same as that for thermal stimulation of traps, while in other materials, the mechanisms may differ. See K. Chakrabarti, V. K. Mathur, J. F. Rhodes and R. J. Abbundi, “Stimulated Luminescence in Rare-Earth-Doped MgS,” Journ. Appl. Phys. 64(3), 1363-1366 (1988).
In theory, optical stimulation is capable not only of depopulating the normal dosimetric traps that are accessible by heating, but also of reaching deeper traps that are not depopulated at the temperatures reached during a typical TLD readout. See L. Botter-Jensen and S. W. S. McKeever, “Optically Stimulated Luminescence Dosimetry Using Natural and Synthetic Materials,” Radiation Protection Dosimetry 65(1-4), 273-280 (1996). Such deep traps can complicate the interpretation of TL readouts since they can communicate with the shallower traps, causing sensitization of the TL signal. This is a problem, for example, with TLD detection using TLD-100 as a dosimetry material. Id. In contrast, since optical stimulation may depopulate these deep traps, OSL dosimetry may provide a more accurate readout of the level of radiation exposure than does TL dosimetry.
In addition, although optical stimulation can completely depopulate all of the populated traps in an OSL dosimeter, complete depopulation of the traps is not a necessary consequence of OSL readout, whereas all thermally accessible traps are permanently read out by heating during a TL readout. In fact, use of a typical continuous wave (cw) or continuously illuminated OSL readout protocol that controls the readout light power and the readout duration often will erase only a small portion of the total trapped charges, providing the opportunity for subsequent OSL or TL readout of the OSL dosimeter.
OSL phosphors such as rare earth-doped alkaline earth sulfides are well known and have been studied extensively. The advantages exhibited by these OSL phosphors include high sensitivity and fast readout. See McKeever 2001, supra. In addition, the bulk temperature of an OSL phosphor is typically not raised due to optical stimulation as long as the amount of the stimulation light that is actually absorbed by the material is small. It follows that optical stimulation of the trapped charges occurs without incurring any of the numerous problems outlined above that are associated with thermal heating of TLD phosphors. For example, the sensitivity of an OSL dosimeter is not reduced during readout by thermal quenching and it is not expected to change as a result of readout. Many conventional OSL phosphors such as the alkaline earth sulfides, however, suffer disadvantages including high fading and poor long-term chemical stability. See M. S. Akselrod, A. C. Lucas, J. C. Polf, and S. W. S. McKeever, “Optically Stimulated Luminescence of Al2O3,” Radiation Measurements 29(3-4), 391-399 (1998). Due to these problems, however, although they have been used in digital radiography applications where fading is not an issue, conventional OSL phosphors have not been adopted for dosimetry applications.
Optically stimulated luminescence dosimeters based on Al2O3:C were developed during the 1990s and introduced to the commercial market by Landauer in the early 2000s. The physical processes in Al2O3:C that are responsible for the OSL dosimeter characteristics are similar to those in other TLD materials, i.e. trapping of electrons at defect centers during exposure to radiation. Exposure to light, at appropriate wavelengths, releases the electrons from the traps and luminescence is emitted via electron-hole recombination.
The advantages of Al2O3:C include high TL sensitivity, cited as 50 times greater than the TL sensitivity of TLD-100, and low fading. The OSL sensitivity is claimed to be 5-10 times higher than its TL sensitivity. The primary disadvantage in the use of Al2O3:C for dosimetry applications is that in order to achieve the necessary OSL sensitivity the optical stimulation must be performed near the peak of the stimulation spectrum. The optically stimulated luminescence of Al2O3:C is centered at about 420 nm while the stimulation light wavelength is typically around 530 nm. See McKeever 2001, supra. The OSL readout system typically uses a pulsed stimulation source and a time-gated detector to prevent the excitation light from interfering with the signal. Id. The dose response has been shown to be “supra linear” for exposures above about 1 Gy. McKeever 1995, supra.
Al2O3:C dosimeters have been coupled to optical fibers and investigated for medical applications such as patient dose verification during radiotherapy. It was found that the radioluminescence sensitivity of the Al2O3:C was dependent on the accumulated dose. See C. E. Andersen, C. J. Marckmann, M. C. Aznar, L. Botter-Jensen, F. Kjaer-Kristoffersen, and J. Medin, “An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al2O3:C,” Radiation Protection Dosimetry 120 (1-4), 7-13 (2006). For this application not only would one need to know the dose rate history in order to calculate the true total dose, but then a complex algorithm would be required to correct for the sensitivity changes that occurred during the measurements. Id.
Al2O3:C is less “tissue equivalent” than LiF-based dosimeters, and therefore displays an over-response for radiation energies below about 120 keV.
The OSL response of most of the commonly used synthetic TLD materials, including metal ion doped crystals of lithium fluoride, calcium sulfate, magnesium borate, and aluminum oxide, has also been previously investigated. See Botter-Jensen, supra. However, this work concluded that “all materials examined produce OSL, to a lesser or greater extent, and under conditions which vary from sample to sample. Generally, most of the synthetic materials showed weak OSL signals that cannot compete with the TL signals and they are of little interest for OSL dosimetry.” They found that the most sensitive material, with the highest potential for OSL dosimetry, was carbon doped aluminum oxide, Al2O3:C. Since this early work, Al2O3:C has been extensively studied and has been commercialized by Landauer and is currently the only OSL dosimeter available for high sensitivity radiation dosimetry applications. See http://www.landauerinc.com/uploadedFiles/Healthcare_and_Education/Products/Dosimeters/LuxelSpecifications.en-US.pdf.
The sensitivity of TLD materials to light exposure is strongly suggestive of the potential utility of the material as an OSL dosimeter and has been previously studied by several groups. See McKeever 2001, supra; see also L. Duggan M. Budzanowskic, K. Przegietkad, N. Reitsemae, J. Wong, and T. Kron, “The light sensitivity of thermoluminescent materials: LiF:Mg,Cu,P, LiF:Mg,Ti and Al2O3:C,” Radiation Measurements 32, 335-342 (2000); and M. Osvay and L. Lembo, “Comparative investigations on UV sensitivity of newly developed LiF TL detectors,” Radiation Protection Dosimetry 4, 227-230 (1993).
However, extreme light sensitivity can be problematic in practice. For example, it is a problem if brief exposure to ambient or room lighting causes significant trap fading, or if exposure to sunlight or UV light results in the population of traps even though the TLD has not been exposed to radiation. This is problematic in that the radiation levels recorded by the dosimeter could be altered by trap fading or sunlight UV population, resulting in an inaccurate reading for the actual exposure seen.
A recent paper by Oster, Horowitz, and Podpalov described what was referred to as “optically stimulated luminescence” from LiF:Mg,Ti, also known as TLD-100 (Thermo Fisher Corporation). See L. Oster, Y. S. Horowitz, and L. Podpalov, “OSL and TL in TLD-100 following alpha and beta irradiation: Application to mixed-field radiation dosimetry,”Radiation Measurements 45, 1130-1133 (2010). In the work described in this paper, samples of TLD-100 were irradiated to very high doses of ˜80 Gy to 120 Gy in order to create color centers in the crystals. The color centers have distinctive absorption bands in the visible portion of the optical spectrum. Excitation with light at a wavelength of about 440 nm results in luminescence from two distinct wavelength bands located at 535 nm and 645 nm. See McKeever 2001, supra.
However, the Horowitz paper incorrectly describes “photoluminescence” as “optically stimulated luminescence.” In the luminescence process described in Horowitz, there is no ionization of the trapping center followed by luminescent recombination, and thus is a photoluminescence process that is different from optically stimulated luminescence as such a process is understood by those skilled in the art. See McKeever 2001, supra. Because the “OSL” signal they report in the paper is actually photoluminescence that occurs as a consequence of radiation damage to the material, the threshold for detection of their “OSL” signal was very high, 80 Gy and the optical method reported, using TLD-100, is not suitable for personal radiation dosimetry applications that require high sensitivity.